Although the potential process efficiency advantages of hot gas desulfurization processes have long been recognized, none of the numerous processes investigated for this purpose has yet achieved commercial success, due to a variety of shortcomings. Several criteria can be identified which would be required by or important to a successful process. First, the process should scrub approximately 90% or more of the sulfur species from the raw gas, in order to meet environmental constraints and also to limit corrosion in downstream equipment. Secondly, the scrub pressure must slightly exceed the desired use pressure of the scrubbed reducing gas. Many major developing uses for reducing gas are at elevated pressure, e.g. 10 atmospheres or higher. The scrub temperature must exceed approximately 1100 K. to assure against carbon deposition. The reducing gases of interest have a temperature in the approximate 950 K. to 1100 K. range below which CO will spontaneously decompose to carbon. Carbon deposition on the sulfur sorbent complicates both scrub and regeneration and also reduces process efficiency. If the hot gas must be cooled sufficiently that CO decomposition is kinetically prevented, it is generally preferable to continue cooling it to where conventional aqueous scrubbing can be conducted.
The regeneration step must also be efficient, i.e. the total utility and energy cost for regeneration must be less than the amount of energy saved due to scrubbing while hot. Thus systems requiring large amounts of steam, air, or other regeneration media per mole of sulfur species scrubbed are generally not viable. Also the regeneration reaction should operate adiabatically with the scrub reaction, due to the difficulty and energy cost of changing the temperature of the sorbent material. The energy cost of regeneration must account for not only the cost of supplying the gaseous regeneration media, but also the cost of converting the sulfur species in the regenerator exhaust to a manageable form--either sulfur or sulfuric acid.
It is also desirable that the scrub and regeneration processes be continuous, in order to avoid the upsets and purge losses associated with batch operations. Sorbent transport is required in continuous systems, and this is greatly facilitated when the scrub and regeneration pressures are approximately equal. Finally the sorbent material must be durable--resistant to dusting, breakage, vapor phase migration, and deactivation due to chemical combination or melting.
Many of the hot gas desulfurization processes investigated to date are summarized in the Environmental Protection Agency report EPA-600/7-79-169, "Hot Gas Cleanup Process", by E. B. Onursal, July 1979, Research Triangle Park, N. C. No Prior art process achieves all of the above attributes. The Ni-S-O system has a melting composition at approximately 950 K., hence its useful temperature is too low, ZnO is limited to scrub temperature below 800.degree. C., apparently due to vapor phase migration and also attrition. The Fe-S-O system has a melting point below 1200 K. It also has a serious problem regarding reducing gas consumption--the regenerated sorbent is either Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4, whereas the spent sorbent is a mixture of FeS, Fe, and some FeO at the high temperatures of interest. Hence this sorbent involves a substantially greater transfer of oxygen into the reducing gas than other sorbents, amounting to a consumption of 1 to 3% of the gas heating value, and hence largely negating the advantages of hot gas cleanup. Copper has a problem of insufficient durability at high temperature, and also is reported to have an undesirably low scrub efficiency at high temperature. MnO has excellent high temperature scrub characteristics. However in the prior art process, MnS must be regenerated with a very dilute oxygen containing gas, making further recovery difficult, and making the volume requirement large. Also the major regeneration produce is Mn.sub.3 O.sub.4, and it is reported that this must be completely reduced by a clean reducing gas to MnO before being restored to scrub service, as otherwise the sulfur content of the product gas would increase significantly until full reduction to MnO was achieved.
The documents referred to in the preparation of this disclosure include U.S. Pat. Nos. 4,164,544 and 3,079,223. The latter patent cites Cu as a sulfur sorbent material over a preferred temperature range of up to 700.degree. C. The patent cites the harmful effect of having Cu.sub.2 O in the regenerated sorbent, and discloses a regeneration method which both prevents the appearance of Cu.sub.2 O in the regenerated sorbent and also allows regeneration at lower temperatures so as to keep the copper solids in better condition. The disclosed method or regenerating Cu.sub.2 S is to react it with somewhat less than twice as much Cu.sub.2 O, whereby Cu.sub.2 S is in stoichiometric excess. The Cu.sub.2 O, is provided by reacting some of the regenerated sorbent with additional Cu.sub.2 O, to eliminate essentially all Cu.sub.2 S from that part, and then reacting the resulting mixture with air.